Genetic disorders of myelin make it possible to study the steps in myelinogenesis. A new area important to these studies in that of virus induced hypomyelination. Congenitally acquired Border disease (BD) virus in sheep causes a selective disordering of myelin synthesis and provides another means of studying early steps in CNS myelinogenesis. This program focuses on studying events that regulate oligodendrocyte differentiation by the use of BD virus in congenitally infected fetal sheep CNS and in a differentiating neural cell culture system. Immunoglobulins are used to identify the major glial and viral antigens in the cell cultures, in CNS tissues and for use in identifying polypeptides in polyacrylamide gels. Immunocytochemical (ICC) studies are done with the light microscope (LM) and the electron microscope (EM). Antigens are co-localized by dual immune labeling using fluorachrome or collodial gold labeled antibodies. The animal studies have determined that although BD virus infects many cell types in the CNS (granule cell neurons, oligodendrocytes and a rare astrocyte), it affects the oligodendrocyte at a critical stage in development causing a delay in myelinogenesis. In the acute infection at mid-gestation the earliest marker for oligodendrocytes, galactocerebroside (Gal-C), and a latter marker myelin basic protein (MBP), are absent. This reduction of myelin components persists in the chronic infection where a differential decrease in three myelin related proteins, myelin associated glycoprotein, MPB and proteolipid protein is noted. To determine how BD virus delays oligodendroglial maturation, a differentiating neural cell culture system was developed from fetal sheep CNS. Proliferating O-2A precursor glial cells expressing only A2B5 could be cultured under conditions in which they either remained O-2A cells with a characteristic bipolar morphology or began to express vimentin and GFAP in addition to A2B5 and acquired the morphology of atrocytes. The affects of BD virus replication on this cell system is under study.